There are two points to the implementation:
* Choice. If I have a separate implementation, my users do not have to be subject to systemd's choices. And I do not either.
* The same implementation will have the same bugs, so in the same way that redundant software has multiple independent implementations, having an independent implementation will avoid the same bugs. It may have different bugs, sure, but my goal would be to test like SQLite and achieve DO-178C certification. Or as close as I could, anyway.